The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature capabilities have been achieved through the development of iron, nickel and cobalt-base superalloys and the use of oxidation-resistant environmental coatings capable of protecting superalloys from oxidation, hot corrosion, etc.
Diffusion aluminide coatings have particularly found widespread use for superalloy components of gas turbine engines. These coatings are generally formed by such methods as diffusing aluminum deposited by chemical vapor deposition (CVD) or slurry coating, or by a diffusion process such as pack cementation, above-pack, or vapor (gas) phase deposition. Diffusion aluminide coatings generally have two distinct zones, the outermost of which is an additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel or cobalt, depending on the substrate material. The MAl intermetallic is the result of deposited aluminum and an outward diffusion of iron, nickel or cobalt from the substrate. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate. During high temperature exposure in air, the additive layer forms a protective aluminum oxide (alumina) scale or layer that inhibits oxidation of the diffusion coating and the underlying substrate.
Components located in certain sections of gas turbine engines, such as the turbine, combustor and augmentor, are often thermally insulated with a ceramic layer in order to reduce their service temperatures, which allows the engine to operate more efficiently at higher temperatures. These coatings, often referred to as thermal barrier coatings (TBC), must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. Coating systems capable of satisfying these requirements typically include a metallic bond coat that adheres the thermal-insulating ceramic layer to the component. In addition to their use as environmental coatings, diffusion aluminide coatings have found wide use as bond coats for TBCs.
Diffusion aluminizing processes generally entail reacting the surface of a component with an aluminum-containing gas composition. In pack cementation processes, the aluminum-containing gas is produced by heating a powder mixture of an aluminum-containing source (donor) material, a carrier (activator) such as an ammonium or alkali metal halide, and an inert filler such as calcined alumina. The ingredients of the powder mixture are mixed and then packed and pressed around the component to be treated, after which the component and powder mixture are heated to a temperature sufficient to vaporize and react the activator with the source material to form a volatile aluminum halide, which then reacts at the surface of the component to form the diffusion aluminide coating.
In contrast to pack processes, vapor phase aluminizing (VPA) processes are able to form a diffusion aluminide coating without the use of an inert filler. In addition, the source material can be an aluminum alloy or an aluminum halide. If the source material is an aluminum halide, a separate activator is not required. Also contrary to pack processes, the source material is placed out of contact with the surface to be aluminized. Similar to pack processes, vapor phase aluminizing is performed at a temperature at which the activator or aluminum halide will vaporize, forming an aluminum halide vapor that reacts at the surface of the component to form the diffusion aluminide coating. VPA processes avoid significant disadvantages of pack processes, such as the use of an inert filler that must be discarded, the use of a source material that is limited to a single use, and the tendency for pack powders to obstruct cooling holes in air-cooled components.
As apparent from the above, pack cementation and vapor phase processes have conventionally required the use of halide carriers or activators. A resulting limitation of these processes is that halides are known to deteriorate any ceramic TBC present on the article being aluminized. Consequently, pack and vapor phase processes have not been widely employed to refurbish components that have existing TBC and require aluminizing of a limited region of the component, such as where TBC has spalled or the interior cooling channels of an air-cooled component. An exception has been a pack cementation process taught by U.S. Pat. No. 5,254,413 to Maricocchi, which employs a source material of about 18 to 45 weight percent aluminum with the balance inert filler. While avoiding the undesirable effect that a halide carrier has on a ceramic TBC, Maricocchi's pack cementation process shares the same disadvantages as those noted for pack cementation processes, namely, the need for an inert filler, the obstruction of cooling holes, and the use of an aluminum powder that must be either discarded or reprocessed after a single use.